Upgrading my own plugin problem

Hi all! I write a simple plugin to manage mailing lists and newsletter in WP. My question is: How to change the table structure during the installation if the table already exixts? Many people download my first version, and i just want to add some column in my second version.
This is the code

if ($wpdb->get_var("show tables like '$table_name_email'") != $table_name_email)
{
add_option("nl_db_version", "1.2");

$sql = "CREATE TABLE " . $table_name_email . " (
id_email int(11) NOT NULL AUTO_INCREMENT,
id_lista int(11) default '1',
contactname varchar(250) default NULL,
email varchar(250) default NULL,
magic_string varchar(250) default NULL,
accepted varchar(1) default ‘n’,
post_id mediumint(9) NULL,
ipaddress VARCHAR(255) NULL,

PRIMARY KEY (id_email),
KEY id_lista (id_lista)

);”;

require_once(ABSPATH . ‘wp-admin/upgrade-functions.php’);
dbDelta($sql);
} else {

//la tabella esiste verifico la versione e aggiorno i campi aggiungendo post_id e ip
$sql=”ALTER TABLE ” . $table_name_email . “
ADD COLUMN post_id INT(9) default NULL,
ADD COLUMN ip_address VARCHAR(55) default NULL ;”;

require_once(ABSPATH . ‘wp-admin/upgrade-functions.php’);
dbDelta($sql);

}

Activating this plugin nothing happens! and i would like just to add 2 columns